1. Field of the Invention
The present invention relates generally to television signal decoding and, more particularly, to a system and method for bad weave detection for inverse telecine.
2. Introduction
Modern high-resolution and high-definition televisions require a method for displaying lower-resolution field-based content (e.g., SDTV, 1080i) in a high quality way. Some types of field-based content can be created through the conversion of a progressive source (such as film) using a process commonly referred to as telecine.
To illustrate the telecine process, consider for example the process of converting 24 frame/s film to 59.94 Hz NTSC video. NTSC is made up of fields, which are made up of every other horizontal scanline. One field contains all the odd numbered scanlines, while the next field contains all the even numbered scanlines. A frame is then the combination of two consecutive fields viewed at the same time. This process is called interlacing.
The conversion of 24 frame/s file to 29.97 frame/s NTSC interlaced video is known as 3:2 pulldown. In this conversion, there are approximately four frames of film for every five frames of NTSC video. These four frames are “stretched” into five by exploiting the interlaced nature of NTSC video. To illustrate this relation, consider the 3:2 pulldown of FIG. 1.
As illustrated, for every NTSC frame, there are actually two fields, one for the odd-numbered lines of the image, designated as the top field (TF), and one for the even-numbered lines, designated as the bottom field (BF). There are, therefore, ten NTSC fields for every four film frames. To accommodate this “stretching” the telecine process alternately places the first film frame across two fields, the second frame across three fields, the third frame across two fields, and the fourth frame across three fields. The cycle repeats itself completely after four film frames have been converted. As illustrated, the TF of the second film frame and the BF of the fourth film frame are duplicate copies.
The particular pulldown method that is applied is based on the type of film and the type of display format (e.g., NTSC, PAL, etc.). As such, various other pulldown methods such as 2:2, 5:5, 6:4, 8:7, etc. can be used for a given application
To display field-based content that is generated by a particular telecine process on a modern television display, it is therefore necessary to recover the original progressive source content such as film. This source can be reconstructed from the field-based content using an inverse telecine process. In this inverse telecine process, it is critical that the correct cadence (e.g., 2:2, 3:2, 5:5, 6:4, 8:7, etc.) of the field-based content be identified so that the correct fields can be weaved together to produce the original progressive source.
If lines from temporally adjacent fields are combined inappropriately, then a “bad weave” can result. These bad weaves can result in an objectionable display. What is needed therefore is a mechanism that enables the proper detection of a cadence of the pulldown.